The best is Megumi Hayashibara she voiced a lot of popular female characters plus her singing voice is great. I loved her as Amatsuka Megumi in Tenshi Na Konamaiki.

I love Aya Hirano for voicing Izumi Konata who is my favorite character in Lucky Star and also she voiced Suzumiya Haruhi, although I don't like that Anime series much I think the quality of her voice is great

I also like Nana Mizuki for her role as Aihara Kotoko in Itazura na Kiss. Her voice quality is amazing and so cute. I love her stammering voice.